Cables on a tray or ladder warm each other, so their ratings come down. AS/NZS 3008.1 gives the grouping factors for them in two tables. How full a tray may be follows from those tables, not from a fill percentage. This page explains how.
The two tables
- Table 23 covers single‑core cables in three‑phase circuits, in trefoil or laid flat.
- Table 24 covers multicore cables.
Each table has rows for the support (an unperforated tray, a perforated tray, a ladder, racks or cleats, a vertical tray), for touching or spaced cables, and for the number of tiers. Its columns are the number of cables or circuits on one tier.
One layer on each tier
The factors hold for one layer of cables on each tier (Note 5 to each table). Cables piled on top of each other are not covered, so a tray is full for rating once its width is used. More cables go on another tier, or on a wider tray.
Note 6 sets the gap between stacked tiers: at least 300 mm for the factors to hold.
Touching or spaced
Touching cables sit side by side. Each multicore cable takes its own overall diameter of width. A trefoil circuit takes two diameters, and a flat circuit three.
Spaced cables have a clear gap of one cable diameter between them (Note 1). Where the sizes differ, the cable tray fill tool uses the largest diameter as the gap, the wider reading.
Spacing doubles the width but raises the factor. Six 16 mm² multicore cables on a perforated tray:
- touching, they take 118.2 mm and read 0.76. Open six cables touching.
- spaced, they take 216.7 mm and read 0.91. Open six cables spaced.
Where the columns stop
The tables print a limited number of columns. Past the last one, no factor is given.
| Table | Cables on a tier | Columns printed |
|---|---|---|
| Table 24 | Multicore, touching | 1, 2, 3, 4, 6 and 9 cables |
| Table 24 | Multicore, spaced | 1, 2, 3, 4 and 6 cables |
| Table 23 | Single‑core circuits | 1, 2 and 3 circuits |
A count between two columns reads the next column up, which gives the lower factor. Seven cables read the 9 column.
So a wide tray can still run out of factors. Four trefoil circuits sit easily on a 450 mm ladder, but Table 23 has no column for four. Open four trefoil circuits on a ladder. Split them over tiers, or check the rating another way.
Tiers
Each tier added lowers the factor or leaves it the same, because the tiers warm each other. The factor is read at the number of tiers and the most cables on any one tier. That makes the fullest tier the one that counts. Ten cables split nine and one read the 9 column. Split five and five, they read the 6 column and get a higher factor. A worked plant room tray, with the factor carried into the cable size, is in Cable tray fill for a plant room: a second tier.
Why there is no fill percentage
A conduit has a space factor: the cables may take only part of its area. AS/NZS 3008.1 prints nothing like that for a tray. The limits are one layer, the gap between tiers, and the columns of the table.
The width you keep free is your choice. It leaves room for later cables. The tool starts at a quarter of the width. Keep more where the job will grow.
The mass
A full tray is heavy. The tool gives the cable mass on each tier from the maker's catalogue. That is the cables alone: check the tray or ladder on its maker's load and span tables.
Carry the factor on
The grouping factor is one of the correction factors on the cable's rating. Enter the same support, tiers and cables on the tier in the cable size calculator, or check one size in the correction factors tool.